1. May 2003
doc.: IEEE /141r3
May, 2005
Project: IEEE P Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: [Ultra-Wideband Peak Power Limits]
Date Submitted: [15 May, 2005]
Source: [Celestino A. Corral, Shahriar Emami and Gregg Rasor] Company [Freescale Semiconductor, Inc.]
Address [6100 Broken Sound Pkwy., N.W., Suite 1, Boca Raton, Florida USA ]
Voice:[ ], FAX: [ ]
Re: [Recent FCC Waiver]
Abstract: [This document provides analytical and theoretical comparison of MB-OFDM and DS-UWB under peak power limited applications.]
Purpose: [For discussion by IEEE TG3a.]
Notice: This document has been prepared to assist the IEEE P It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P
Celestino A. Corral et al., Freescale
Jai Balakrishnan et al., Texas Instruments

2. Ultra-Wideband Peak Power Limits
May, 2005
Ultra-Wideband Peak Power Limits
Celestino A. Corral, Shahriar Emami and Gregg Rasor
Freescale Semiconductor, Inc.
6100 Broken Sound Parkway., N.W., Suite 1
Boca Raton, Florida USA
May 17, 2005
Celestino A. Corral et al., Freescale

3. May, 2005
Motivation
Goal: To provide a comparison between DS-UWB and MB-OFDM for peak-limited applications under the recent FCC waiver.
Note: Recent FCC waiver is technology-neutral. Devices can be measured under “normal” operating conditions. These conditions can include hopping or gating.
Approach: Consider DS-UWB and MB-OFDM waveforms under average- and peak-power measurements. Emphasis is on peak-to-average power ratio of waveforms.
Additionally: Provide peak-power headroom levels for actual implementation considerations.
Celestino A. Corral et al., Freescale

4. Average Power Measurements
May, 2005
Average Power Measurements
Radiated
Waveform
Spectrum analyzers measure average value of the total signal power quantized within resolution bandwidth by making a fixed number of measurements and computing a corrected average figure of power density normalized to that bandwidth.
Celestino A. Corral et al., Freescale

5. Average Power Measurements
May, 2005
Average Power Measurements
Resolution bandwidth filter
Block Diagram of Typical Spectrum Analyzer
For FCC emission measurements, the resolution bandwidth is 1 MHz with 1 msec integration time for the RMS power and resulting EIRP. Resolution bandwidth is 50 MHz for peak power measurements.
Celestino A. Corral et al., Freescale

6. May, 2005
Gated Signals
gated signal t
Gating allows greater power transmissions over narrower time intervals. This power can be used to improve SNR, SIR or range. Limit is now peak power.
ungated signal T
Celestino A. Corral et al., Freescale

7. Peak Power Measurements
May, 2005
Peak Power Measurements
50 MHz
1 MHz
key determinant for peak-power levels
Minimize PAPR to achieve more headroom in peak power levels
Peak power measurements actually made with spectrum analyzer on “peak hold” capturing over a long time period (several minutes).
Celestino A. Corral et al., Freescale

8. Direct-Sequence UWB Sinusoidal carrier, PAPR = 3 dB
May 2003
doc.: IEEE /141r3
May, 2005
Direct-Sequence UWB
Sinusoidal carrier, PAPR = 3 dB
Data spread by chipping code
Upconverted to desired freq.
Shaped by RRC filter with a = 0.3.
Spectral BW = 1.5 GHz. Waveform has 40% fractional bandwidth between 3.1 and 4.6 GHz and consequently good fading resilience.
0.26 ns
code
4.1 GHz
adjust
RRC
Filter
data
Celestino A. Corral et al., Freescale
Jai Balakrishnan et al., Texas Instruments

9. What Spectrum Analyzer Measures
May, 2005
What Spectrum Analyzer Measures
DS-UWB Waveform
Signal over air has 5.5 dB PAPR
1 MHz
Filter
50 MHz
Filter
DS-UWB has 8.5 dB PAPR (ungated) in 50 MHz filter.
Celestino A. Corral et al., Freescale

10. Worst-Case PAPR of MB-OFDM
May, 2005
Worst-Case PAPR of MB-OFDM
Subcarrier spacing is MHz.
In 50 MHz resolution bandwidth this corresponds to 12 subcarriers.
Worst-case PAPR is 10log(12)=10.8 dB.
Above occurs even if MB-OFDM waveform is clipped to 9 dB PAPR.
If we consider that hopping contributes 5.8 dB additional PAPR for 3 hops, the total worst-case PAPR is 16.6 dB.
As a result, we have about 7.7 dB headroom for MB-OFDM.
50 MHz
Celestino A. Corral et al., Freescale

11. How Often Does This Happen?
May, 2005
How Often Does This Happen?
QPSK
Constellation
90o 0o
180o
270o
Celestino A. Corral et al., Freescale

12. Impact of Filtering Operation
May, 2005
Impact of Filtering Operation
Worst-Case OFDM Symbol
12 Subcarriers
Filter Impulse Response
(50 MHz)
Output of Filter
(Convolution)
Pulse width is about 8% of the length of OFDM symbol.
pulse width
The filter impulse response is very narrow relative to the OFDM waveform, so convolution results in OFDM symbol and PAPR is conserved.
Celestino A. Corral et al., Freescale

13. What Spectrum Analyzer Measures
May, 2005
What Spectrum Analyzer Measures
Multi-Band OFDM Waveform
Signal over air has 9 dB PAPR
1 MHz
Filter
50 MHz
Filter
On average, peak power is dBm and PAPR is 15 dB. Worst-case PAPR is 16.6 dB and peak-power is -7.7 dBm.
Celestino A. Corral et al., Freescale

14. May, 2005
Summary of Results
Parameter
DS-UWB
MB-OFDM
PAPR at transmit pin
3.0 dB
9.0 dB
PAPR over air after pulse shaping
5.5 dB
PAPR at output of 50 MHz filter
8.5 dB
16.6 dB
Peak power in 50 MHz bandwidth
-15.8 dBm
-7.7 dBm
Thus, DS-UWB has 8.1 dB more headroom than MB-OFDM. This can be employed to overcome cable losses, antenna losses, etc. DS-UWB has a net 15.8 dB headroom for exploiting gating.
Celestino A. Corral et al., Freescale

15. May, 2005
Conclusions
Multi-band OFDM (MB-OFDM), even if clipped to a 9 dB peak-to-average over the air, can still result in up to a 16.6 dB PAPR in a 50 MHz bandwidth.
For MB-OFDM, the 16.6 dB PAPR in a 50 MHz bandwidth is due to 10.8 dB of signal PAPR for 12 subcarriers captured in that bandwidth and 5.8 dB of PAPR due to the 3-hop sequence.
DS-UWB has a nominal 5.5 dB PAPR after pulse shaping.
The PAPR of ungated DS-UWB in a 50 MHz bandwidth is 8.5 dB.
DS-UWB has 15.8 dB maximum headroom for transmission which can be exploited for gated signals. This corresponds with an approximate duty cycle of 3%.
Consequently, DS-UWB has 8.1 dB more headroom than MB-OFDM for overcoming cable, filter and antenna losses.
Celestino A. Corral et al., Freescale